This invention relates to radiation detectors and, in particular, to electromagnetic radiation detectors known as bolometers.
A typical bolometer, as shown in FIG. 1, includes an absorber 12 of electromagnetic radiation, a resistive thermometer 14 and a heat sink 16. The absorber 12 intercepts incident electromagnetic radiation and converts it into heat, thereby raising the temperature of the absorber 12. The thermometer 14 is in thermal contact with the absorber 12 and measures its temperature rise. The thermometer 14 is normally selected to be of a material which has a temperature-dependent resistivity. Therefore, by connecting (and biasing) the thermometer in an electric circuit, it produces a voltage indicative of the temperature of the absorber. This voltage is the "bolometer" output, which may be calibrated to indicate the absorbed radiation power or, alternatively, to indicate incident radiation power.
In a steady state condition, the heat flow out of the absorber is equal to the incident radiation absorbed by it. The heat flows out along several paths. As shown in FIG. 1, some heat flows by conduction through a path that physically connects the absorber to the heat sink 16, which is a body that is maintained at relatively fixed temperature. Also, as shown in FIG. 1, some heat is radiated out of the absorber and thermometer by radiation conduction to the surroundings. Heat flow through an ambient gas can be minimized by operation in a vacuum.
The thermal conductance, G.sub.heat sink, from the absorber to the heat sink is defined as the ratio of steady state heat flow between these elements, divided by their temperature difference, in the absence of any other heat flows in the system. Similarly, a thermal conductance G.sub.rad may be defined as the ratio of steady state heat flow between the absorber and surroundings, divided by their temperature difference, in the absence of any other heat flows in the system.
The total thermal conductance G of the bolometer is the combined thermal conductance through all paths of heat flow from the absorber and thermometer to the heat sink and surroundings. It is given by: EQU G=G.sub.heat sink +G.sub.rad (1)
where G.sub.heat sink and G.sub.rad are as defined above. When the total thermal conductance G is made smaller, the temperature rise of the absorber will be greater for a given amount of absorbed radiation, and more signal will be available to the thermometer. It is therefore an object of this invention to reduce the total thermal conductance, G, which is defined in equation 1. It is also an object of this invention to accomplish this while maintaining the fastest time response of the bolometer, i.e. the shortest time constant.
The thermal conductance due to radiation G.sub.rad is given by:
G.sub.rad =4.eta..sigma.aT.sub.0.sup.4 (2)
where .eta. is the emissivity, .sigma. is the Stefan-Boltzmann constant, "a" is the total area of the absorber and other bolometer parts whose temperature can vary, and T.sub.0 is the temperature of the surroundings.
The time constant .tau. of the bolometer characterizes the length of time to respond to a change in incident power level. Ignoring the effects of self-heating due to the thermometer bias, it is given by: EQU .tau.=C/G (3)
where C is the thermal capacitance of the bolometer.
Responsivity is defined as the ratio of the bolometer output to the power incident on the bolometer, and may be expressed in units of Volts/Watt. Ignoring the effects of self-heating, the zero-frequency responsivity increases in inverse proportionality to the total thermal conductance G. Thus, a smaller value of G is desirable for achieving a larger value of responsivity. A smaller value of G also generally results in improved sensitivity.
According to Eq. (1), the value of G can be reduced by decreasing one of its components, G.sub.heat sink. This may be done by making a long structure, with narrow cross section, as the link between the absorber and heat sink. The other component contributing to the total thermal conductance, G.sub.rad is then a limit on how large the responsivity can be. Equation (2) shows that, for a given surrounding temperature T.sub.o and emissivity .eta., the bolometer area "a" will be a limiting factor in responsivity. With the other parameters already specified, minimizing the area "a" will be the principal means for achieving maximum responsivity. The area of the bolometer is also critical for determining the time constant. In general, the thermal capacitance C decreases with the bolometer area. Therefore, according to Eq. (3), the time constant also decreases with decreasing area. A smaller area "a" is therefore desirable for achieving a faster time response, as well as increasing the responsivity.
Taking into account the heating of the bolometer due to thermometer bias current introduces a correction to Eq. (3). However, it is still true that a smaller bolometer area results in a faster time response and permits a higher responsivity to be obtained.
FIGS. 2 and 3 show prior art bolometers employing absorbers based on antennas. The bolometers in FIG. 2a and FIG. 2b include physical structures with a low thermal conductance between the entire absorber and the heat sink. The dotted line indicates the perimeter of a structure with low thermal conductance to the heat sink, such as a membrane. All parts within the dotted line are located on this structure. In FIG. 2a,incident radiation, received by the antenna, is delivered to a thin-film resistor at the antenna feed. Joule-heating within the resistor converts the radiation to heat. An adjacent resistive thermometer (which may be part of a bridge circuit) measures the temperature rise. In FIG. 2b, which is a different version of the configuration shown in FIG. 2a,the resistor at the antenna feed plays two roles; thermometer and converter of radiation to heat. One way of reducing the total thermal conductance of the bolometers in FIG. 2 is by suspending the absorber and thermometer on a membrane. The perimeter of the membrane makes direct contact to a thicker structure (not shown) which functions as the heat sink. The relatively small cross-section for heat flow through the membrane results in a small value of the thermal conductance (G.sub.heat sink) through the membrane. However, this strategy is limited by the other component of thermal conductance, G.sub.rad. The area of the bolometer that enters into G.sub.rad, which may be calculated in accordance with equation 2, includes the entire area of the antenna, and cannot be made any smaller than the antenna. Thus, there is a limit on how small G.sub.rad can be, which in turn limits the total thermal conductance G.
This constraint on the bolometer designs of FIGS. 2a and 2b is especially limiting for bolometers that detect millimeter-wave radiation. Antennas for these wavelengths are millimeters in length and a fraction of a millimeter in width, resulting in a thermal radiation conductance G.sub.rad that is quite large, particularly when the surroundings are at room-temperature.
FIGS. 3a and 3b also show two configurations of bolometers with antenna absorbers. In these configurations, the thermometer (but not the antenna) has a low thermal conductance to the heat sink. The parts within the dotted line are located on a structure, such as an "air-bridge", with low thermal conductance to the heat sink. The parts outside of the dotted line are located on the heat sink. Thus, in contrast to FIGS. 2a and 2b, the antenna is located on the heat sink. In FIG. 3a, only the resistive termination at the antenna feed, and an adjacent thermometer are suspended on a low thermal conductance structure. In FIG. 3b, the resistive termination and thermometer are combined in a single resistor. This configuration has been implemented in an "air-bridge" configuration, with the resistor on a small bridge standing above a silicon substrate. It has also been implemented with a thermocouple-type thermometer.
However, there is a drawback to the configurations of FIG. 3 which limits the highest responsivity that can be achieved. The electrical impedances of the antenna and its resistive termination must be substantially equal, to achieve optimal coupling of radiation from the former to the latter. Typical antenna impedances are in the range of hundreds of Ohms. However, this constraint on the electrical impedance of the termination resistance also constrains its thermal conductance. This is because of the Weideman-Franz law, according to which the electrical and thermal impedances are related through a proportionality constant that is independent of the material, for a wide range of conductive materials. The thermal impedance of the termination is therefore specified by its electrical impedance. This is a limit on the thermal conductance, and the bolometer responsivity. It is an object of the invention to reduce these constraints.